The development biology of the homothallic ascomycete Aspergillus nidulans will be studied as a model eukaryotic system, with emphasis on the regulation of gene expression during development. We are taking two complementary approaches: The first is a mutational analysis of the asexual and sexual sporulation pathways. Previous work in this laboratory has identified mutants of genes (preinduction genes) which are implicated in the acquisition of competence for asexual development. Molecular methods are proposed to test directly whether these preinduction genes control the expression of postinduction genes at the transcriptional level. A genetic approach will be used to determine whether developmental genes are clustered as suggested by molecular analysis of cloned genomic segments. The sexual sporulation pathway will be similarly analyzed using a recently developed efficient screening method for mutants blocked in sexual development. The second approach will concentrate on one or two developmentally regulated proteins that are present in large amounts and easily isolable. One such protein is the enzyme laccase (p-diphenol oxidase) which is required for formation of the green conidial pigment and without which the conidia are yellow. Many deficient mutants are available, and we have purified the enzyme and made antibody against it. Putative control mutants have been isolated, the properties of which suggest that copper may be required for transcription of the laccase structural gene. This possibility will be tested directly by assaying laccase mRNA by means of a clone of the laccase structural gene. Other laccase control mutants will be sought including laccase constitutive mutants. A strategy is proposed for achieving DNA transformation of A. nidulans which will allow the full potentials of this developmental system to be exploited.